third_party/WebKit/Source/core/layout/ScrollAlignment.cpp

/*
 * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
 *
 * Portions are Copyright (C) 1998 Netscape Communications Corporation.
 *
 * Other contributors:
 *   Robert O'Callahan <roc+@cs.cmu.edu>
 *   David Baron <dbaron@fas.harvard.edu>
 *   Christian Biesinger <cbiesinger@web.de>
 *   Randall Jesup <rjesup@wgate.com>
 *   Roland Mainz <roland.mainz@informatik.med.uni-giessen.de>
 *   Josh Soref <timeless@mac.com>
 *   Boris Zbarsky <bzbarsky@mit.edu>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
 *
 * Alternatively, the contents of this file may be used under the terms
 * of either the Mozilla Public License Version 1.1, found at
 * http://www.mozilla.org/MPL/ (the "MPL") or the GNU General Public
 * License Version 2.0, found at http://www.fsf.org/copyleft/gpl.html
 * (the "GPL"), in which case the provisions of the MPL or the GPL are
 * applicable instead of those above.  If you wish to allow use of your
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 * deletingthe provisions above and replace them with the notice and
 * other provisions required by the MPL or the GPL, as the case may be.
 * If you do not delete the provisions above, a recipient may use your
 * version of this file under any of the LGPL, the MPL or the GPL.
 */

#include "core/layout/ScrollAlignment.h"

#include "platform/geometry/LayoutRect.h"

namespace blink {

const ScrollAlignment ScrollAlignment::alignCenterIfNeeded = {
    ScrollAlignmentNoScroll, ScrollAlignmentCenter, ScrollAlignmentClosestEdge};
const ScrollAlignment ScrollAlignment::alignToEdgeIfNeeded = {
    ScrollAlignmentNoScroll, ScrollAlignmentClosestEdge,
    ScrollAlignmentClosestEdge};
const ScrollAlignment ScrollAlignment::alignCenterAlways = {
    ScrollAlignmentCenter, ScrollAlignmentCenter, ScrollAlignmentCenter};
const ScrollAlignment ScrollAlignment::alignTopAlways = {
    ScrollAlignmentTop, ScrollAlignmentTop, ScrollAlignmentTop};
const ScrollAlignment ScrollAlignment::alignBottomAlways = {
    ScrollAlignmentBottom, ScrollAlignmentBottom, ScrollAlignmentBottom};

#define MIN_INTERSECT_FOR_REVEAL 32

LayoutRect ScrollAlignment::getRectToExpose(const LayoutRect& visibleRect,
                                            const LayoutRect& exposeRect,
                                            const ScrollAlignment& alignX,
                                            const ScrollAlignment& alignY) {
  // Prevent degenerate cases by giving the visible rect a minimum non-0 size.
  LayoutRect nonZeroVisibleRect(visibleRect);
  LayoutUnit minimumLayoutUnit;
  minimumLayoutUnit.setRawValue(1);
  if (nonZeroVisibleRect.width() == LayoutUnit())
    nonZeroVisibleRect.setWidth(minimumLayoutUnit);
  if (nonZeroVisibleRect.height() == LayoutUnit())
    nonZeroVisibleRect.setHeight(minimumLayoutUnit);

  // Determine the appropriate X behavior.
  ScrollAlignmentBehavior scrollX;
  LayoutRect exposeRectX(exposeRect.x(), nonZeroVisibleRect.y(),
                         exposeRect.width(), nonZeroVisibleRect.height());
  LayoutUnit intersectWidth =
      intersection(nonZeroVisibleRect, exposeRectX).width();
  if (intersectWidth == exposeRect.width() ||
      intersectWidth >= MIN_INTERSECT_FOR_REVEAL) {
    // If the rectangle is fully visible, use the specified visible behavior.
    // If the rectangle is partially visible, but over a certain threshold,
    // then treat it as fully visible to avoid unnecessary horizontal scrolling
    scrollX = getVisibleBehavior(alignX);
  } else if (intersectWidth == nonZeroVisibleRect.width()) {
    // If the rect is bigger than the visible area, don't bother trying to
    // center. Other alignments will work.
    scrollX = getVisibleBehavior(alignX);
    if (scrollX == ScrollAlignmentCenter)
      scrollX = ScrollAlignmentNoScroll;
  } else if (intersectWidth > 0) {
    // If the rectangle is partially visible, but not above the minimum
    // threshold, use the specified partial behavior
    scrollX = getPartialBehavior(alignX);
  } else {
    scrollX = getHiddenBehavior(alignX);
  }

  if (scrollX == ScrollAlignmentClosestEdge) {
    // Closest edge is the right in two cases:
    // (1) exposeRect to the right of and smaller than nonZeroVisibleRect
    // (2) exposeRect to the left of and larger than nonZeroVisibleRect
    if ((exposeRect.maxX() > nonZeroVisibleRect.maxX() &&
         exposeRect.width() < nonZeroVisibleRect.width()) ||
        (exposeRect.maxX() < nonZeroVisibleRect.maxX() &&
         exposeRect.width() > nonZeroVisibleRect.width())) {
      scrollX = ScrollAlignmentRight;
    }
  }

  // Given the X behavior, compute the X coordinate.
  LayoutUnit x;
  if (scrollX == ScrollAlignmentNoScroll)
    x = nonZeroVisibleRect.x();
  else if (scrollX == ScrollAlignmentRight)
    x = exposeRect.maxX() - nonZeroVisibleRect.width();
  else if (scrollX == ScrollAlignmentCenter)
    x = exposeRect.x() + (exposeRect.width() - nonZeroVisibleRect.width()) / 2;
  else
    x = exposeRect.x();

  // Determine the appropriate Y behavior.
  ScrollAlignmentBehavior scrollY;
  LayoutRect exposeRectY(nonZeroVisibleRect.x(), exposeRect.y(),
                         nonZeroVisibleRect.width(), exposeRect.height());
  LayoutUnit intersectHeight =
      intersection(nonZeroVisibleRect, exposeRectY).height();
  if (intersectHeight == exposeRect.height()) {
    // If the rectangle is fully visible, use the specified visible behavior.
    scrollY = getVisibleBehavior(alignY);
  } else if (intersectHeight == nonZeroVisibleRect.height()) {
    // If the rect is bigger than the visible area, don't bother trying to
    // center. Other alignments will work.
    scrollY = getVisibleBehavior(alignY);
    if (scrollY == ScrollAlignmentCenter)
      scrollY = ScrollAlignmentNoScroll;
  } else if (intersectHeight > 0) {
    // If the rectangle is partially visible, use the specified partial behavior
    scrollY = getPartialBehavior(alignY);
  } else {
    scrollY = getHiddenBehavior(alignY);
  }

  if (scrollY == ScrollAlignmentClosestEdge) {
    // Closest edge is the bottom in two cases:
    // (1) exposeRect below and smaller than nonZeroVisibleRect
    // (2) exposeRect above and larger than nonZeroVisibleRect
    if ((exposeRect.maxY() > nonZeroVisibleRect.maxY() &&
         exposeRect.height() < nonZeroVisibleRect.height()) ||
        (exposeRect.maxY() < nonZeroVisibleRect.maxY() &&
         exposeRect.height() > nonZeroVisibleRect.height())) {
      scrollY = ScrollAlignmentBottom;
    }
  }

  // Given the Y behavior, compute the Y coordinate.
  LayoutUnit y;
  if (scrollY == ScrollAlignmentNoScroll)
    y = nonZeroVisibleRect.y();
  else if (scrollY == ScrollAlignmentBottom)
    y = exposeRect.maxY() - nonZeroVisibleRect.height();
  else if (scrollY == ScrollAlignmentCenter)
    y = exposeRect.y() +
        (exposeRect.height() - nonZeroVisibleRect.height()) / 2;
  else
    y = exposeRect.y();

  return LayoutRect(LayoutPoint(x, y), nonZeroVisibleRect.size());
}

}  // namespace blink